iPhone: Understanding tethering data usage
iOS 3.1 or later offers the ability to view tethering data usage in Settings > General > Usage.
Certain wireless carriers do not separate cellular network usage and tethering data usage. In these instances, Cellular Network Data usage (see image above) reflects your combined cellular network usage and tethering data usage.
When using iOS 3.1, Tethering Data may appear even if your iPhone*, carrier, or wireless-data plan does not offer Internet tethering. In addition, in certain instances 1KB of Tethering Data usage may be displayed even though data tethering is not available for your iPhone. No actual data has been transferred over the cellular data network, and this does not indicate tethering availability for your iPhone, carrier, or wireless-data plan. See Locating iPhone wireless carriers for tethering availability by carrier.
To resolve this issue, use iTunes to update your iPhone to iOS 3.1.2 or later.
* Internet tethering is not available on the original iPhone. For more information, see iPhone 3G: System requirements for Internet tethering.